I've worked on a lot of games that worked similarly to Frank C's description. There's a discrete physics frame that's running at 60, 90, or 120 fps. The graphics render whenever possible. We did this all the time for PC games, where performance of the system was highly variable.

After all, future iPhones could come along with faster processors and graphics. So I think it's a good way to go.

I'm not as far as you are yet. Is there a callbacks you get when a frame has rendered? Is there a callback when it's been displayed? Or our we filling a queue with frames?